Various beamforming methods for wireless systems equipped with multiple transmit and multiple receive antennas are known in the art. At present, space time coding schemes are currently proposed for multiple antenna systems.
FIG. 1, taken from U.S. Pat. No. 6,584,302, assigned to the assignee hereof, presents a transceiver that has a group of antenna elements 200-204 for transmitting and receiving. The transceiver is usually a base station but it can also be subscriber equipment. The antenna with several elements can be an antenna array or some other kind of cluster of antenna elements. Referring to receiver 260, each signal received from each antenna 200-204 enters RF-means 206-210 that convert the radio frequency signal to baseband signals in a known manner. The signals are digitized in A/D-converters 212-216. The digital baseband signals are multiplied by coefficients W1-WM that form the shape of the beam of the antenna in multipliers 218-222. The coefficients W1-WM are digital complex numbers. The receiver searches for the values of the coefficients W1-WM that produce the best reception. Antenna responses are calculated in an antenna response unit 224 for each antenna element. The antenna responses are ranked and a subset of the set of the antenna responses is selected in rank and select unit 226.
A response of an antenna element is similar to an impulse response and is calculated by using correlation. In the correlation a known pseudo-random spreading code is correlated with the received signal L times. L is the number of paths of the multipath propagated signal. After calculating one correlation value the spreading code is shifted by time a difference ΔT, which can be the same as the duration of a chip.
In the transmitter 262 the subset comprising at least one antenna response is fed to a coefficient unit 230 that calculates the coefficients a1-aM for each antenna element 200-204 transmitting a signal. The signal to be transmitted is multiplied by the coefficients using the multipliers 232-236. The signal weighted by the coefficients a1-aM is then converted to an analog signal by D/A-converters 238-242. After that, the analog signals are converted to radio frequency signals in RF-means 244-248 and the radio frequency signals are transmitted by the antenna elements 200-204.
In this environment performance gains are sought. Space time coding schemes in multiple transmit and multiple receive antenna environments result in complex receiver circuitry. It is known that power allocation at the transmitter antennas according to the water-filling algorithm can result in significant improvement in signal to noise ratio. Improvements in signal to noise ratio, however, come at a cost when using the water filling algorithm. First, a feed back channel is required between the receiver and transmitter to provide information to the transmitter concerning channel state. Second, the water-filling algorithm heretofore required complete channel information. In practical systems, only partial channel information may be available at the transmitter due to the limited nature of feedback resources.
Hence, those skilled in the art desire feedback-based transmission schemes for cases where partial channel information is available at the transmitter. Transmission schemes for single receive antenna systems utilizing quantized channel information have been developed, but are not satisfactory.
For example, antenna systems based on unit rank beamforming designs that use finite rate feedback of channel characteristics provide significant advantages over space time coding implementations both from the perspective of reduced receiver complexity and signal-to-noise ratio. Nonetheless, these advantages exist only over a relatively limited range of transmission rates. Thus, those skilled in the art desire beamforming designs using finite rate feedback of channel characteristics that preserve the advantages of the waterfilling algorithm as transmission rates increase. The maintenance of the performance advantage as transmission rates increase should not come at the cost of unduly increased receiver complexity that would provide no advantage over space time code implementations.
Higher rank beamforming schemes are markedly different from unit rank beamforming, both in the nature of the feedback information required and the required encoder/decoder. Unit rank beamformers employ a scalar encoder and decoder, resulting in low complexity. Higher rank beamforming schemes require vector encoder/decoder for good performance and, as a result, are more complex. Those skilled in the art nonetheless desire higher-rank beamforming methods that result in decoding circuitry which is still less complex then that required by space-time coding methods.